Doped, pyrogenically prepared oxides

ABSTRACT

Doped, pyrogenically prepared oxides of metals and/or non-metals which are doped with one or more doping components in an amount of 0.00001 to 20 wt. %. The doping component may be a metal and/or non-metal or an oxide and/or a salt of a metal and/or a non-metal. The BET surface area of the doped oxide may be between 5 and 600 m 2 /g. The doped pyrogenically prepared oxides of metals and/or non-metals are prepared by adding an aerosol which contains an aqueous solution of a metal and/or non-metal to the gas mixture during the flame hydrolysis of vaporizable compounds of metals and/or non-metals.

INTRODUCTION AND BACKGROUND

The present invention relates to doped, pyrogenically prepared oxides, aprocess for their preparation and their use.

It is known that pyrogenically prepared oxides can be coated with metalsalts or metal oxides by mixing the pyrogenically prepared oxides withaqueous solutions of metal salts and then drying and/or calcining.

Products prepared in this way have the disadvantages a) that the dopingsubstance is not homogeneously distributed in the entire primaryparticle or b) that, depending on the type of doping, inhomogeneitiesmay occur during mixing. Thus, after doping and calcining, the primaryparticles of the doping substance may separate out and be present withmuch larger diameters than the primary particles of pyrogenic oxides.

It is therefore an object of the invention to achieve homogeneous dopingof pyrogenically prepared oxides with another substance while at thesame time avoiding problems of the prior art, and more particularly, toavoid the presence of separate primary particles of the doping substanceor oxides of the doping substance alongside primary particles of thepyrogenically prepared oxide.

SUMMARY OF THE INVENTION

The above as well as other objects are obtained by the present inventionin the form of doped, pyrogenically prepared oxides of metals and/ornon-metals wherein the basic components are pyrogenically preparedoxides of metals and/or non-metals, prepared using flame hydrolysistechniques, which are doped with at least one doping component at0.00001 to 20 wt. %, wherein the doping amount may preferably be in therange 1 to 10,000 ppm, and the doping component is a non-metal and/or ametal or a non-metal salt and/or a metal salt or an oxide of a metaland/or a non-metal, and the BET surface area of the doped oxides isbetween 5 and 600 m²/g.

Another feature of the invention is a process for preparing doped,pyrogenically prepared oxides of metals and/or non-metals. In carryingout the process, an aerosol is fed into a flame, such as is used in aknown manner to prepare pyrogenic oxides by flame hydrolysis and whereinthis aerosol is homogeneously mixed with the gas mixture for flameoxidation or flame hydrolysis prior to reaction. The aerosol/gas mixtureis allowed to react in the flame and the resulting doped pyrogenicallyprepared oxides are separated from the gas stream in a known manner. Asalt solution or suspension which contains the components of thesubstance to be doped, which may be a metal salt or a non-metal salt(metalloid salt) or mixtures of both or a suspension of an insolublemetal compound or non-metal (metalloid) compound, is used as thestarting material for the aerosol. The aerosol is prepared bynebulization using a two-fluid nozzle or using an aerosol generator,preferably by the ultrasonic method.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood with reference to the drawings,wherein:

FIG. 1 is a schematic diagram of an apparatus suitable for practicingthe present invention;

FIG. 2 is an EM photograph of pyrogenic silicon made without doping; and

FIG. 3 is an EM photograph of pyrogenic silicon made with doping inaccordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In carrying out the present invention, aerosol technology is used tofeed into a flame in order to prepare pyrogenic oxides by flamehydrolysis.

The aerosol may be introduced in a preferred embodiment of the inventionby means of a device such as the one shown in FIG. 1. According to FIG.1, the main part of the apparatus is a burner 1 of known construction,such as is conventionally used for preparing pyrogenic oxides. In thiscase, the pipes for gas and aerosol introduction may also beinterchanged.

The burner 1 consists of a central tube 2 which discharges into a nozzle3, out of which the main gas stream flows into the combustion chamber 8and is there burned off. The inner nozzle is surrounded by the furtherannular nozzle 4 (mantle nozzle), out of which flows mantle- orsecondary-hydrogen to prevent caking.

According to the invention, a centrally located axial tube 5 is locatedinside central tube 2 and terminates a few centimeters upstream of thenozzle 3 of the central tube 2. The aerosol is fed into the axial tube5, whereby the aerosol gas stream from the axial tube 5 is homogeneouslymixed with the gas stream from the central tube 2 over the last sectionof the central tube 2. The central tube conveys air, hydrogen and, forexample, silicon tetrachloride for the pyrolysis reaction.

The aerosol is produced in an aerosol generator 6 (ultrasonicnebulizer). An aqueous salt solution 9 located in the generator 6contains the metal or non-metal as a salt in dissolved ordispersed/suspended form and is used as the aerosol starting material.The aerosol produced by the aerosol generator 6 is passed through theheating zone 7 by means of a carrier gas stream 10, whereupon the waterevaporates and small, finely distributed salt crystals remain in the gasphase. The flame pipe 11 can be cooled with cooling water 12. Secondaryair can be introduced through part 13 into the chamber 8.

In a further embodiment of the invention, the aerosol may be introducedusing an annular nozzle, which may be arranged at any angle, preferablyat right angles, to the main gas stream.

The basic components which may be used are the non-metals/metalsaluminum, niobium, titanium, tungsten, zirconium, germanium, boronand/or silicon.

The doping components which are used may be metals and/or non-metals andtheir compounds, provided they are soluble in or can be suspended in aliquid solution. In a preferred embodiment, compounds of transitionmetals and/or noble metals may be used for this purpose. The term“transition metals” is used herein in its art-recognized meaning.

By way of example, cerium and potassium salts may be used as dopingcomponents.

The flame hydrolysis process for preparing pyrogenic oxides is knownfrom Ullmann's Encyclopedia of Industrial Chemistry, 4th ed., vol. 21,page 464, the disclosure of which is relied on and incorporated hereinby reference.

Due to the fine distribution of the doping component in the aerosol, andalso the high temperatures (1,000 to 2,400° C.) during subsequent flamehydrolysis, during which the doping components are possibly furtherreduced in size and/or melted, the doping medium is finely distributedin the gas phase during the initial stages of production of thepyrogenic oxide, so that homogeneous incorporation of the dopingcomponent in the pyrogenically prepared oxide is possible.

Using the process according to the invention, it is possible to dope allknown pyrogenically prepared oxides (e.g., SiO₂, TiO₂, Al₂O₃, B₂O₃,ZrO₂, GeO₂, WO₃, Nb₂O₅) with other metal or metal oxides or non metal ornon metal (metalloid) oxides or mixtures thereof.

The process according to the invention has several advantages: Theaggregate or agglomerate structure of the pyrogenic oxide can beinfluenced by the choice of doping components. Furthermore, the pH ofthe pyrogenic oxide can be affected.

Catalytically active substances (e.g., cerium or noble metals) which areused as doping components can be distributed almost homogeneously in thepyrogenically prepared oxide.

Phase conversion of pyrogenically prepared oxides, for example fromanatase to rutile in pyrogenically prepared titanium oxide, can beaffected by doping.

Using the process according to the invention, combinations of propertiesof pyrogenically prepared oxides which have hitherto not been available,or available only with great difficulty, i.e. for example in processesrequiring several steps, can be achieved.

Pyrogenically prepared oxides of metals and/or non-metals, dopedaccording to the invention, can be used as fillers, as supportmaterials, as catalytically active substances, as starting materials forpreparing dispersions, as polishing materials for polishing metal orsilicon wafers in the electrical industry, as ceramic substrates, in theelectronics industry (CMP applications), in the cosmetics industry, asadditives in the silicone and rubber industry, to adjust the rheology ofliquid systems, for heat-resistant stabilization purposes, in thelacquer industry, as a heat insulation material, and the like.

The invention also provides a device for performing the processaccording to the invention which is characterized in that an additionaltube for introducing the aerosol is arranged, preferably axially, in aburner of the structure known for preparing pyrogenic oxides, whereinthe tube terminates upstream of the burner nozzle.

EXAMPLES

The burner arrangement used in examples 1 to 4 is shown schematically inFIG. 1.

Example 1 (no doping) 4.44 kg/h of SiCl₄ are evaporated at about 130° C.and introduced into the central tube of the burner. 3 Nm³/h of primaryhydrogen and 8.0 Nm³/h of air are also fed to the central tube. The gasmixture flows out of the inner nozzle of the burner and burns in thecombustion chamber and the water-cooled flame tube connected in seriestherewith. 0.5 NM³/h of mantle or secondary hydrogen are fed to themantle nozzle which surrounds the central nozzle, in order to preventcaking of the nozzle. An additional 12 Nm³/h of secondary air are fed tothe combustion chamber.

The aerosol flows out of the axial tube into the central tube. Theaerosol consists of water vapor which has been produced in an amount of195 g/h by ultrasonic nebulization of pure distilled water in theaerosol generator.

The nebulized water vapor is passed through a heated pipe with theassistance of a carrier gas of about 0.5 Nm³/h of air, wherein theaerosol is converted into gas at a temperature of about 180° C.

At the mouth of the burner (nozzle 3), the temperature of the gasmixture (SiCl₄/air/hydrogen, water vapor and water aerosol) is 150° C.

The reaction gases and the resulting pyrogenic silica are passed undersuction through a cooling system, by applying a reduced pressure to theflame tube, and thus cooled to about 100 to 160° C. The solid isseparated from the vent gas stream in a filter or a cyclone.

The silica is produced as a white, finely divided powder. In a furtherstep, adhering residues of hydrochloric acid are removed from the silicaby treating it with water vapor-containing air at elevated temperature.

The BET surface area of the pyrogenic silica is 150 m²/g.

The production parameters are given in Table 1. Further analytical datarelating to the pyrogenic silica obtained are given in Table 2.

Example 2: (Doping with cerium)

The same procedure is used as described in Example 1: 4.44 kg/h of SiCl₄are evaporated at about 130° C. and introduced into the central tube ofthe burner. 3 Nm³/h of primary hydrogen and 8.0 Nm³/h of air are alsosupplied to the central tube. The gas mixture flows out of the innerburner nozzle and burns in the combustion chamber and the water-cooledflame tube connected in series therewith. In the mantle nozzle whichsurrounds the central nozzle, 0.5 Nm³/h of mantle or secondary hydrogenare supplied in order to prevent caking. An additional 12 Nm³/h ofsecondary air are supplied to the combustion chamber.

The aerosol flows out of the axial tube into the central tube. Theaerosol is a cerium salt aerosol which is produced in an amount of 210g/h by ultrasonic nebulization of a 5% aqueous cerium(III) chloridesolution in the aerosol generator.

The cerium salt aerosol is passed through a heated pipe with theassistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas and a salt crystal aerosol at temperatures around180° C.

At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

The reaction gases and the resulting pyrogenically prepared silica,doped with cerium, are removed under suction via a cooling system byapplying a reduced pressure and thus cooled to about 100 to 160° C. Thesolid is separated from the gas stream in a filter or cyclone.

The doped, pyrogenically prepared silica is produced as a white, finelydivided powder. In a further step, adhering hydrochloric acid residuesare removed from the silica by treatment with water vapor-containing airat elevated temperatures.

The BET surface area of the doped, pyrogenically prepared silica is 143m²/g.

The production parameters are given in Table 1. Further analytical datafor the pyrogenic silica obtained are given in Table 2.

Example 3 (no doping) 4.44 kg/h of SiCl₄ are evaporated at about 130° C.and transferred to the central tube in the burner. 3 Nm³/h of primaryhydrogen and 8.7 Nm³/h of air are also fed through the central tube. Thegas mixture flows out of the inner nozzle of the burner and burns in thecombustion chamber and the water-cooled flame tube connected in seriestherewith. In the mantle nozzle which surrounds the central nozzle, 0.5Nm³/h of mantle or secondary hydrogen are supplied in order to preventcaking. An additional 12 Nm³/h of secondary air are supplied to thecombustion chamber.

The aerosol flows out of the axial tube into the central tube. Theaerosol consists of water vapor which is produced in an amount of 210g/h by ultrasonic nebulization of pure distilled water in the aerosolgenerator.

The aerosol is passed through a heated pipe with the assistance of about0.5 Nm³/h of air as carrier gas, wherein the aerosol is converted into agas at temperatures around 180° C.

At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, water vapor or water aerosol) is 180° C.

The reaction gases and the resulting pyrogenic silica are removed undersuction via a cooling system by applying a reduced pressure and thuscooled to about 100 to 160° C. The solid is separated from the gasstream in a filter or cyclone.

The silica is produced as a white, finely divided powder. In a furtherstep, adhering hydrochloric acid residues are removed from the silica bytreatment with water vapor-containing air at elevated temperature.

The BET surface area of the pyrogenic silica is 215 m²/g.

The production parameters are given in Table 1. Further analytical datafor the pyrogenic silica obtained are given in Table 2.

Example 4: (Doping with cerium)

The same procedure is used as described in Example 1: 4.44 kg/h of SiCl₄are evaporated at about 130° C. and introduced into the central tube ofthe burner. 3 Nm³/h of primary hydrogen and 8.7 Nm³/h of air are alsosupplied to the central tube. The gas mixture flows out of the innerburner nozzle and burns in the combustion chamber and the water-cooledflame tube connected in series therewith.

In the mantle nozzle which surrounds the central nozzle, 0.5 Nm³/h ofmantle or secondary hydrogen are supplied in order to prevent caking.

An additional 12 Nm³/h of secondary air are supplied to the combustionchamber.

The aerosol flows out of the axial tube into the central tube. Theaerosol is a cerium salt aerosol which has been produced in an amount of205 g/h by ultrasonic nebulization of a 5% aqueous cerium(III) chloridesolution in the aerosol generator.

The cerium salt aerosol is passed through a heated pipe with theassistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas and a salt crystal aerosol at temperatures around180°C.

At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

The reaction gases and the resulting pyrogenically prepared silica,doped with cerium, are removed under suction via a cooling system byapplying a reduced pressure and thus cooled to about 100 to 160° C. Thesolid is separated from the gas stream in a filter or cyclone.

The doped, pyrogenic silica is produced as a white, finely dividedpowder. In a further step, adhering hydrochloric acid residues areremoved from the pyrogenic silica by treatment with watervapor-containing air at elevated temperatures.

The BET surface area of the doped, pyrogenic silica is 217 m²/g.

The production parameters are given in Table 1. Further analytical datafor the pyrogenic silica obtained are given in Table 2.

Example 5: (Doping with potassium salts)

The same procedure is used as described in Example 1, wherein a 0.5%aqueous potassium chloride solution is used as salt solution. 4.44 kg/hof SiCl₄ are evaporated at about 130° C. and introduced into the centraltube of the burner. 3 Nm³/h of primary hydrogen and 8.7 Nm³/h of air arealso supplied to the central tube. The gas mixture flows out of theinner burner nozzle and burns in the combustion chamber and thewater-cooled flame tube connected in series therewith.

In the mantle nozzle which surrounds the central nozzle, 0.5 Nm³/h ofmantle or secondary hydrogen are supplied in order to prevent caking.

The aerosol flows out of the axial tube into the central tube. Theaerosol is a potassium salt aerosol which has been produced in an amountof 215 g/h by ultrasonic nebulization of a 0.5% aqueous potassiumchloride solution in the aerosol generator.

The potassium salt aerosol is passed through a heated pipe with theassistance of 0.5 Nm³/h of air as carrier gas, wherein the aerosol isconverted into a gas and a salt crystal aerosol at temperatures around180° C.

At the mouth of the burner, the temperature of the gas mixture(SiCl₄/air/hydrogen, aerosol) is 180° C.

The reaction gases and the resulting pyrogenically prepared silica,doped with potassium, are removed under suction via a cooling system byapplying a reduced pressure and the particle/gas stream is thus cooledto about 100 to 160° C. The solid is separated from the gas stream in afilter or cyclone.

The doped, pyrogenically prepared silica is produced as a white, finelydivided powder. In a further step, adhering hydrochloric acid residuesare removed from the silica by treatment with water vapor-containing airat elevated temperatures.

The BET surface area of the doped, pyrogenically prepared silica is 199m²/g.

The production parameters are given in Table 1.

Further analytical data for the pyrogenic silica obtained are given inTable 2.

TABLE 1 EXPERIMENTAL CONDITIONS DURING THE PREPARATION OF DOPEDPYROGENIC SILICAS Prim. Sec. H₂ N₂ Gas Aerosol Air SiCl₄ air air H₂ coremantle mantle temp. Salt amount aeros. BET No. kg/h Nm³/h Nm³/h Nm³/hNm³/h Nm³/h ° C. soln. kg/h Nm³/h m²/g Doping with cerium salt andcomparison examples 1 4.44 8.0 12 3 0.5 0.3 150 only H₂O 0.195 0.5 150 24.44 8.0 12 3 0.5 0.3 180 5% CeCl₃ 0.210 0.5 143 3 4.44 8.7 12 3 0.5 0.3180 only H₂O 0.210 0.5 215 4 4.44 8.7 12 3 0.5 0.3 180 5% CeCl₃ 0.2050.5 217 Doping with potassium salt and comparison example 3 4.44 8.7 123 0.5 0.3 180 only H₂O 0.210 0.5 215 5 4.44 8.7 12 3 0.5 0.3 180 0.5%KCl 0.215 0.5 199 Notes: Prim. air = amount of air in central tube; sec.air = secondary air; H₂ core = hydrogen in central tube; Gas temp. = gastemperature at the nozzle in the central tube; Aerosol amount = massflow of salt solution converted into aerosol form; Air aerosol = carriergas (air) in the aerosol.

TABLE 2 ANALYTICAL DATA FOR SAMPLES OBTAINED ACCORDING TO EXAMPLES 1 TO5 Ce K Cl Grindo Sedi- pH Comp. Thick. BET wt. wt. conc. LOD LOI Clmeter vol. Effic- 4% b. d. Lupoda No. [m²/g] μg/g μg/g ppm wt. % wt. %ppm μm vol % iency sus. g/l l [mPas] Doping with cerium salt andcomparison examples 1 150 — 0.19 1.29 18 0 697 3.98 27 1745 2 143 1860<5 0.09 1.33 20 0 690 3.93 26 1990 3 215  84 <5 45 0.27 1.87 45 18 11422 4.00 25 3390 4 217 2350 <5 112 0.22 2.23 112 40 50 548 3.67 29 3680Doping with potassium salt and comparison example 3 215 <5 45 0.27 1.8745 18 11 422 4.00 25 3390 5 199 300 55 0.32 1.86 55 60 50 451 4.83 322575 Notes: Cerium content as Ce in μg/g (ppm); Potassium content as Kin μg/g; LOD = loss on drying (2 h at 105° C., based on DIN/ISO 787/II,ASTM D 280, JIS K 5101/21); LOI = loss on ignition (2 h at 1000° C.,based on DIN 55921, ASTM D 1208, JIS K 5101/23, with ref. to substancedried for 2 h at 105° C.); Grindometer = Grindometer value; Sedi-vol. =sediment volume; Efficiency = turbidity measurement: the method # ofefficiency determination (turbidity measurement) is described in PatentDE 44 00 0170; the suspension prepared by the same method is used fordetermining sediment volume after standing for a further 5 minutes;Compacted bulk density based on DIN/ISO 787/IX, JIS K 5101/18 (notsieved). Thickening in polyester reference system: described in EP-A 0015 315.

FIG. 2 shows an EM photograph of pyrogenic silica prepared in accordancewith Example 3 (no doping).

FIG. 3 shows an EM photograph of pyrogenic silica prepared in accordancewith Example 4 (doped with cerium salt).

It can be seen that the aggregate and agglomerate structure is modifiedwhen doped with cerium salt. Larger cohesive structures are producedwith doping.

The analytical data for silica in accordance with Example 4, as comparedwith that for silica in accordance with Example 3, shows an increasedsediment volume and a greatly increased efficiency value. This alsoindicates enlargement of the aggregate or agglomerate structure.

It should be noted that comparable results are obtained in accordancewith the present invention when a noble metal is used in place of ceriumin Example 4. Similarly, when silica is replaced with aluminum oxide,niobium oxide, germanium oxide or boron oxide, comparable results wouldbe obtained.

Furthermore, using silica doped with cerium in accordance with theinvention, a clear improvement in thickening effect is produced inunsaturated polyester resins.

Further variations and modifications will be apparent to those skilledin the art from the foregoing and are intended to be encompassed by theclaims appended hereto. German priority application 195 50 500.3 isrelied on and incorporated herein by reference.

What is claimed is:
 1. A process of producing a doped, pyrogenic oxidewhich is doped using an aerosol, the process comprising: providing anaerosol comprised of at least one of a solution and a suspension of adoping substance selected from the group consisting of a metal salt, anon-metal salt, a metalloid salt, and a mixture thereof; conducting theaerosol through a heating zone by means of a carrier gas stream tothereby evaporate at least a portion of the aerosol; mixing theevaporated aerosol in the carrier gas stream with a combustible gasstream which comprises at least one of an evaporated solution and anevaporated suspension of a substance to be doped selected from the groupconsisting of a metal salt, a non-metal salt, a metalloid salt, and amixture thereof, to thereby create a combustible gas stream mixture;reacting the combustible gas stream mixture in a flame; and separatingthe resulting doped, pyrogenic oxide from the reacted gas streammixture.
 2. The process of producing a doped, pyrogenic oxide accordingto claim 1, wherein the substance to be doped comprises at least onemember selected from the group consisting of aluminum, niobium,titanium, tungsten, zirconium, germanium, boron and silicon.
 3. Theprocess of producing a doped, pyrogenic oxide according to claim 1wherein the at least one of a solution and a suspension of a dopingsubstance comprises at least one of a transition metal and a noblemetal.
 4. The process of producing a doped, pyrogenic oxide according toclaim 1, wherein the resulting doped, pyrogenic oxide comprises at leastone member selected from the group consisting of SiO₂, TiO₂, Al₂O₃,B₂O₃, ZrO₂, GeO₂, WO₃, and Nb₂O₅.
 5. The process of producing a doped,pyrogenic oxide according to claim 1, wherein the at least one of asolution and a suspension of a substance to be doped comprises SiCl₄,and wherein the at least one of a solution and a suspension of a dopingsubstance comprises cerium (III) chloride.
 6. The process of producing adoped, pyrogenic oxide according to claim 1, wherein the process bywhich the doped, pyrogenic oxide is produced comprises a further step oftreating the resulting doped, pyrogenic oxide with air containing watervapor.
 7. The process of producing a doped, pyrogenic oxide according toclaim 1, wherein the at least one of a solution and a suspension of adoping substance comprises a 5% aqueous cerium (III) chloride solution.8. The process of producing a doped, pyrogenic oxide according to claim1, wherein the at least one of a solution and a suspension of a dopingsubstance comprises a 0.5% aqueous potassium chloride solution.
 9. Theprocess of producing a doped, pyrogenic oxide according to claim 1,wherein the resulting doped, pyrogenic oxide is doped with at least onedoping component in an amount of from 0.00001 to 20% by weight.
 10. Theprocess of producing a doped, pyrogenic oxide according to claim 1,wherein the resulting doped, pyrogenic oxide has a BET surface area offrom 5 to 600 m²/g.
 11. The process of producing a doped, pyrogenicoxide according to claim 1, wherein the doping component comprise atleast one member selected from the group consisting of cerium, a ceriumsalt, and an oxide of cerium.
 12. The process of producing a doped,pyrogenic oxide according to claim 1, wherein the doping componentcomprises at least one member selected from the group consisting of anoble metal, a noble metal salt and a noble metal oxide.
 13. The processof producing a doped, pyrogenic oxide according to claim 1, wherein thedoping component comprises at least one member selected from the groupconsisting of a transition series element, a transition series elementsalt and a transition series oxide.
 14. The process of producing adoped, pyrogenic oxide according to claim 1, wherein the providing anaerosol is via nebulization.
 15. The process of producing a doped,pyrogenic oxide according to claim 1, wherein the nebulization isultrasonic nebulization.
 16. The process of producing a doped, pyrogenicoxide according to claim 1, wherein the doping component comprises atleast one of potassium, a potassium salt, and an oxide of potassium. 17.The process of producing a doped, pyrogenic oxide according to claim 1,wherein an amount of the doping substance in the resulting doped,pyrogenic oxide is from 1 to 10,000 ppm.
 18. A method of modifyingaggregate and agglomerate structure of pyrogenic silica doped withcerium salt, comprising: providing a precursor of the pyrogenic silicaand a precursor of the cerium salt as at least one of a solution and asuspension of the precursors; and presenting the at least one of asolution and a suspension of the precursors in aerosol form prior to astep of reacting the precursors in a flame.